This is the Hydraulic Throw-out Bearing / Concentric Slave Cylinder (CSC) assembly found inside the bellhousing of the transmission.
These are found on all manual 6MT Zs and Gs equipped with the HR or VHR engine.
This part is prone to failure when upgrading to a performance clutch or simply due to excessive heat during track use. Z1 Motorsports offers a bolt on kit to eliminate the stock internal slave cylinder and replace it with an easily replaceable external slave cylinder. Z1 CSC Delete Kit